Re: [Xen-devel] [PATCH] trace: fix build with gcc9

[George Dunlap <George.Dunlap@xxxxxxxxxx>]

> On Mar 8, 2019, at 7:11 AM, Jan Beulich <jbeulich@xxxxxxxx> wrote:
> 
>>>> George Dunlap <george.dunlap@xxxxxxxxxx> 03/07/19 1:02 PM >>>
>> On 3/7/19 10:34 AM, Jan Beulich wrote:
>>> While I've not observed this myself, gcc 9 (imo validly) reportedly may
>>> complain
>>> 
>>> trace.c: In function '__trace_hypercall':
>>> trace.c:826:19: error: taking address of packed member of 'struct 
>>> <anonymous>' may result in an unaligned pointer value
>>> [-Werror=address-of-packed-member]
>>>  826 |     uint32_t *a = d.args;
>> 
>> Wait, is this saying that in this case (i.e., with a single uint32_t
>> before args), you *do* get an unaligned pointer value, or just that if
>> the struct changes in the future that the pointer value may become
>> un-aligned?
> 
> With the __packed attribute, the compiler _could_ place the structure at
> a misaligned slot on the stack. It has got nothing to do with struct layout
> afaict. Note how the diagnostic says "may", not "does”.

That’s interesting — so is there no way to specify that the struct should be 
aligned but the individual elements not?

If that’s the case, what about doing something like the attached instead?  It 
avoids introducing a not-very-obvious BUILD_BUG_ON(), and also I think makes 
the algorithm a (tiny) bit easier to follow.  (And if the BUILD_BUG_ON() ever 
triggered, we’d probably end up having to do something like this anyway.)

 -George

Attachment: 0001-xen-trace-Fix-gcc9-alignment-warning.patch
Description: 0001-xen-trace-Fix-gcc9-alignment-warning.patch

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